This invention relates to the field of lead loss compensation for fiber optic sensors, and more particularly to a dual wavelength technique for compensating losses caused by phenomenon other than the sensed condition.
With the rapid growth of fiber optic-based communications, increasing attention is being paid to the use of fiber optic sensors for detecting such phenomena as acoustic waves, rotation rates, acceleration, pressure, magnetic and electric fields, temperature, stress and strain, etc. Such fiber optic sensors are normally based on a transducer mechanism that depends upon modification of the polarization of light passing through the sensor. Thus, the polarization of the light beam is modulated in accordance with the sensed phenomenon. A typical configuration for such a sensor includes a light source for providing a light beam, fiber optic cables for transmitting the light beam, a photo transducer which modulates the light beam in accordance with the externally applied phenomenon, additional fiber optic cables for transmitting the modulated light beam to photo detectors where the modulation of the light beam may be decoded, and a processor which outputs a usable signal indicating the magnitude of the sensed phenomenon.
A significant drawback to the typical photo sensor configuration is that losses caused by signal attenuation in the fiber optic cables and their connectors is significant compared to the signal level of the light beam. In addition, power fluctuations in the light source itself also may overshadow the signal level detected by the photo detectors. Turning to FIG. 1, it can be seen that the intensity of the signal detected by the photo detectors may be hidden by fiber optic lead and connector losses. FIG. 1 is directed to a photo transducer which detects stress. The X axis depicts increasing amounts of stress applied to the photo transducer, while the Y axis depicts the intensity of the optical signal detected by the photo detectors. Assuming that the detected optical power is I.sub.s, it is uncertain whether the stress applied to the photo transducer is S.sub.1, S.sub.2, or S.sub.3. The stress is highly dependent upon the lead/connector losses of the fiber optic cable system. Such losses are inherent in any fiber optic photo sensor device employing fiber optic cables, and these losses severely restrict the potential applications for such a device. Even where such a device may be applied, the accuracy of the measured optical signal is highly suspect.
One solution to this problem would be to calibrate the lead/connector losses before each measurement. This solution would provide minimal improvement since such lead/connector losses can vary over time. In addition, such a calibration scheme would fail to compensate the detected signal for power fluctuations in the light source itself. In addition, such constant recalibration would consume a great deal of time for the minimal benefits achieved thereby. Therefore, a more precise solution to this problem must be achieved before fiber optic sensing systems achieve wide acceptability in the sensor field.
One such solution is proposed in U.S. Pat. No. 4,368,645 to Glenn et al. Glenn et al discloses an optical pressure sensor which can compensate for some (but not all) lead/connector losses and light source power fluctuations. The device according to Glenn et al provides a light source which generates a light beam that is input into a fiber optic cable. The fiber optic cable directs the source light beam to a lens which collimates the light beam. The collimated light beam is then passed through a linear polarizer and a quarter wave plate to circularly polarize the collimated light beam. The circularly polarized light beam is then introduced into a photoelastic transducer which modulates the polarized light beam in accordance with pressure applied to the photoelastic transducer. The modulated light beam is then directed to a polarizing beam splitter which splits the modulated light beam into first and second components. Each separate component is focused by a lens into a separate fiber optic cable. These fiber optic cables then direct the first and second components to photo detector devices for detecting the intensity of the first and second components. The dependence of the intensities of the two components on the pressure applied to the photoelastic transducer permits the measurement of that pressure in a manner that has quadratic error dependence on optical misalignment. The difference in the intensities of the two components is then divided by the sum of the intensities of the two components to eliminate lead/connector losses in the fiber optic cable leading up to the transducer. However, such a scheme does nothing to compensate for transmission losses from the output of the transducer to the photo detector devices. Thus, the device according to Glenn et al is still highly susceptible to fiber optic lead/connector losses. If such losses in the output fiber optic cables are significant, the device according to Glenn et al will not function properly.
Another solution for compensating the sensitivity variations in fiber optic cables and the drift of the light source and photo detectors is proposed in U.S. Pat. No. 4,493,995 to Adolfsson et al. In Adolffson et al, an optical source provides a source light beam at a single wavelength which is used as a carrier wave. This carrier wave is then modulated at one or more lower frequencies in accordance with the phenomenon sensed by the photo transducer. The material in the photo transducer responds differently depending upon the modulation frequency of the light beam, not upon the carrier frequency (wavelength) itself. The response of the photo transducer material at the different modulation frequencies can then be used to determine the sensed phenomenon independently of lead and connector losses. However, such a device is complex and expensive due to the necessity of modulating the carrier wave. In addition, demodulation electronics are required, also increasing the complexity and cost of this device.
Therefore, what is needed is an inexpensive yet precise solution to compensating photo sensor systems for fiber optic lead/connector losses and power fluctuations in the light source.